Methods for passivation of protectively coated steel sheets are sufficiently well-known. Examples of these include chromating and phosphating (FP2092090B1). A common feature of all of these methods, however, is the removal or pickling of the natural or native oxide layer and replacement of it by another passivation layer. Such passivation layers can contribute, among other things, to improving the adhesion to an organic coating, e.g. of paints. In a subsequent processing of the protectively coated steel sheet, it is impossible to avoid a partial removal of the passivation layer. In addition to requiring increased cleaning, this can also result in altered process parameters in the subsequent processing zones, which can negatively affect the reproducibility of the subsequent processing.
Alternatively, WO2006045570A1 proposes increasing this adhesive capacity of the protectively coated steel sheet by modifying the natural oxide layer, without pickling this natural oxide layer as a result. Thus in the continuous process of providing the protective coating of the steel sheet, a cooling of the steel sheet with an aqueous composition or coolant is carried out, which is intended to improve the natural oxide layer of the protective coating, which contains Zn, Mg, and Al, for example. The aqueous composition can have soluble salts added to it to protect the natural oxide layer or phosphates to stabilize the sheet surface. But such a method cannot result in a remarkable increase in the adhesive capacity.